DISPERSION OF FRESH-WATER FISHES. 131 



the White-fish, the Pike, and to arctic and sub- 

 arctic species generally, Behring Strait have evi- 

 dently proved no serious obstacle to diffusion ; 

 and it is not unlikely that much of the close re- 

 semblance of the fresh-water faunae of northern 

 Europe, Asia, and North America is due to this 

 fact. To attempt to decide from which side the 

 first migration came in regard to each group of 

 fishes might be interesting ; but without a wider 

 range of facts than is now in our possession, such 

 attempts would be mere guesswork and without 

 value. The interlocking of the fish-faunae of Asia 

 and North America presents, however, a number of 

 interesting problems, for numerous migrations in 

 both directions have doubtless taken place. 



I could go on indefinitely with the discussion of 

 special cases, each more or less interesting or sug- 

 gestive in itself, but the general conclusion is in all 

 cases the same. The present distribution of fishes 

 is the result of the long-continued action of forces 

 still in operation. The species have entered our 

 waters in many invasions from the Old World or 

 from the sea. Each species has been subjected to 

 the various influences implied in the term " natural 

 selection," and under varying conditions its repre- 

 sentatives have undergone many different modifi- 

 cations. Each of the six hundred species we now 

 know may be conceived as making every year in- 

 roads on territory occupied by other species. If 

 these colonies are able to hold their own in the 

 struggle for possession, they will multiply in the 

 new conditions, and the range of the species be- 

 comes widened. If the surroundings are different, 



